Low-firing temperature method for producing Al2O3 bodies having enhanced chemical resistance

ABSTRACT

The present invention includes a method for producing high-alumina bodies with superior chemical properties at reduced sintering temperatures. One form of the method includes the steps of providing an alumina powder precursor, adding about 2 wt. % magnesia powder precursor and about 2 wt. % titania powder precursor, mixing the resultant green powder precursor, pressing a green body from the green powder precursor, removing residual moisture and organic material from the green body, and firing the green body to about cone  13.    
     The resulting high-alumina body has a substantially uniformly sized grain structure, is resistant to dissolution in molten aluminum, and has superior resistance to chemical attack over substantially the entire pH range.

REFERENCE TO RELATED APPLICATION

[0001] The present application is a utility application based uponProvisional application Ser. No. 60/188,506, filed Mar. 10, 2000.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates generally to a method for forminghigh-alumina bodies, and, more particularly, to a method for sinteringhigh alumina bodies having superior properties and at reducedtemperatures.

BACKGROUND OF THE INVENTION

[0003] Alumina (also known as Al₂O₃ or corundum) is a useful andubiquitous ceramic material. Alumina is a very hard crystallinematerial. It has a structure that may be described as a hexagonalclose-pack array of oxygen atoms with metal atoms in two-thirds of theoctahedrally coordinated interstices. Each metal atom is thuscoordinated by six oxygen atoms, each of which has four metal neighbors(6:4 coordination). Alumina products include abrasives, insulators,structural members, refractory bricks, electronic substrates, and tools.Alumina is stable, hard, lightweight, and wear resistant, making itattractive for such applications as seal rings, air bearings, electricalinsulators, valves, thread guides, and the like, as well as the ceramicreinforcing component in metal matrix composites.

[0004] Alumina is produced on an industrial scale using the BayerProcess to separate ferric oxide, silica and aluminum oxides. Bauxiteore is ground finely then treated with sodium hydroxide (NaOH) in aniron autoclave at an elevated temperature. The alumina dissolves assodium aluminate via the equation:

Al₂O₃+2NaOH→2NaAlO₂+H₂O

[0005] The silica dissolves to form sodium silicate but the ferricoxide, being insoluble, is filtered off. Carbon dioxide is then passedthrough the solution, decomposing the sodium aluminate (NaAlO₂) to formaluminum hydroxide and sodium carbonate:

2NaAlO₂+CO₂→Na₂CO₃+⇓2Al(OH)₃

[0006] The aluminum hydroxide is separated by filtration and calcined at1000° C. or higher, when it loses its water of constitution, yieldingalumina:

2Al(OH)₃→Al₂O₃+3H₂O

[0007] Pure crystalline alumina is a very inert substance and resistsmost aqueous acids and alkalis. It is more practical to use eitheralkaline (NaOH) or acidic (KHSO₄, KHF₂, etc) melts. Concentrated boilingsulfuric acid also can be used as an etchant.

[0008] In order to produce useful bodies, alumina must be densified orsintered. Sintering is the process in which a compact of a crystallinepowder is heat treated to form a single coherent solid. The drivingforce for sintering is the reduction in the free surface energy of thesystem. This is accomplished by a combination of two processes, theconversion of small particles into fewer larger ones (particle and graingrowth) and coarsening, or the replacement of the gas\solid interface bya lower energy solid\solid interface (densification). This process ismodeled in three stages:

[0009] Initial—the individual particles are bonded together by thegrowth of necks between the particles and a grain boundary forms at thejunction of the two particles.

[0010] Intermediate—characterized by interconnected networks ofparticles and pores.

[0011] Final—the structure consists of space-filling polyhedra andisolated pores.

[0012] The kinetics of sintering tend to be temperature sensitive, suchthat an increase in sintering temperature generally substantiallyaccelerates the sintering process. In industrial applications, while anincrease in sintering temperature decreases sintering time and increasesthroughput, the economic gains therefrom are offset by increased fuelcosts and decreased furnace life (since higher firing temperaturesresult in more rapid degradation of both the furnace refractorystructure and heating elements.) Therefore, an economically optimumsintering temperature is one that best balances gains from throughputwith losses from fuel and furnace wear and tear.

[0013] The sintering of alumina at temperatures above 1600° C. isrequired to achieve a high density, and alumina is commonly sintered inthe temperature range of 1700-1800° C., since higher temperaturespromote more rapid sintering. Sintered alumina bodies reflect theproperties of the constituent alumina crystallites or grains, such thatthey are hard, tough, substantially inert, and resistant to chemicalattack. Mechanical and/or chemical failure of sintered alumina bodiesusually occurs as a grain boundary phenomenon. Since the grainboundaries usually contain porosity and a glassy phase, a sinteredalumina body is not as hard, tough, inert, and/or chemically resistantas a comparable single crystal alumina body.

[0014] There is therefore a need for a technique for decreasing thesintering temperature of alumina without sacrificing throughput(increasing the sintering time) or quality. There is also a need forproducing sintered alumina bodies having bulk physical characteristicscloser to those of single-crystal alumina. The present inventionaddresses these needs.

SUMMARY OF THE INVENTION

[0015] One form of the present invention relates to a process for thelow-temperature sintering of high-alumina bodies. The high-aluminabodies so produced have a substantial resistance to dissolution inmolten aluminum. The effective sintering temperature for a givensintering time required to achieve substantially full densification weredecreased through the addition of quantities a of magnesia-titaniamixture to the alumina precursor powders. The resulting substantiallyfully dense high-alumina bodies exhibited superior resistance tochemical attack over a broad range of pH and temperature conditions.

[0016] One object of the present invention is to provide a method forproducing substantially dense high-alumina bodies at lower sinteringtemperatures when sintered for comparable times. Related objects andadvantages will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a flow chart schematically representing the processingsteps of the present invention.

[0018]FIG. 2 is a table illustrating some of the physical properties ofhigh-alumina material made by the process of FIG. 1.

[0019]FIG. 3 is a table presenting the results of exposure of thehigh-alumina material made by the process of FIG. 1 to various hostilechemical environments.

[0020]FIG. 4 is a table presenting some properties of thermal spraycoatings of the high-alumina material of FIG. 1.

[0021]FIG. 5 is a table presenting some properties of metal matrixcomposites made from the low-fired high-alumina material of FIG. 1

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated device, and such furtherapplications of the principles of the invention as illustrated thereinbeing contemplated as would normally occur to one skilled in the art towhich the invention relates.

Known Methods of Decreasing the Firing Temperature of Alumina

[0023] To promote the rapid densification of Al₂O₃, additives such asCaO, MgO and TiO₂, as well as titanates of baria and strontia, havefrequently been used. The effect of these additives is sensitive tocertain experimental or production procedures, including the fabricationhistory of the Al₂O₃ powder, the amount of additives (especially MgO),the sintering temperature, type and concentration of impurities, and soon. The effectiveness of known methods of densification is also afunction of the purity alumina and additives. Densification generallyincreases as a function of purity.

[0024] MgO is known in the art to act to retard grain growth or, moreprecisely, to restrict the mobility of grain boundaries. Two categoriesof grain boundaries can be distinguished: those that intersect pores andare therefore active in densification (type A) and those that areentirely connected to other grain boundaries (type B). The existence oftype B boundaries is due to inhomogeneties in the original powdercompaction of the green body. Densely packed regions of the greencompact undergo local densification, leading to the development ofdense, pore-free regions upon firing in an otherwise porousmicrostructure. These dense regions will be better able to support graingrowth because of the drag exerted by porosity is absent.

[0025] The effect of MgO is limited to the increase in the grainboundary area. MgO alone has no effect on the pore surface area. Theraising of the grain boundary area can be interpreted as being due tothe inhibition of grain growth in the densified regions, i.e. the grainsremain small. The function of the additive is to restrain grain growthin the densely packed regions until the less densely packed regions havean opportunity to densify. MgO can be thought of as acting as ahomogenizer of the microstructure, in that MgO smoothes out theconsequences of inhomogeneity.

[0026] The mechanism by which MgO aids in densification has been asource of contention in the known art. Generally, two mechanisms havebeen considered: pore mobility and grain boundary mobility. Thecontention arises from (including but not limited to) the nature andamount of impurities, experimental regimen and sintering atmosphere. Onepossible densification mechanism is that MgO increases the surfacediffusion kinetics and thus increases pore mobility. The resultant highpore mobility keeps pores on the migrating grain boundaries during thefinal stages of sintering. Other mechanisms such as solute segregationat the grain boundaries and second phase pinning of grain boundaries mayhave been proven untenable, but the data may not be conclusive and theyare being mentioned because they describe interesting sinteringphenomenon.

[0027] Another sintering aid known to be effective in the densificationof alumina is TiO₂. Additions of TiO₂ to alumina have resulted in morerapid sintering relative to pure alumina. For additions of titania asthe only sintering additive, the rate of initial sintering generallyincreases approximately exponentially with titania concentration up to apercentage beyond which the rate of sintering remained constant ordecreased slightly. The concentration, which produces the maximum rateof sintering, is thought to be the solubility of TiO₂ in Al₂O₃. Foralumina particles larger than 2 μm in initial stage sinteringexperiments with temperatures of 1520° C. and 1582° C., the kineticprocess was mainly grain boundary diffusion. For smaller particles (lessthan 1 μm) in initial stage sintering experiments with temperaturesranging from 1150° C. to above 1400° C., volume diffusion dominated. Forparticles with sizes between the two, sintering occurred by acombination of the two kinetic mechanisms. It should be noted that theabove details are for initial stage sintering wherein a maximum densityof about 85% was achieved.

[0028] Fine-grained alumina bodies of about 95% theoretical density wereachieved by sintering green alumina bodies with 2 wt. % additions of lowmelting point additives at 1400° C. Silicate additions were used becausethey form a liquid phase during the firing cycle. Silicate fluxes wereprepared using MgO and CaO and in long firing regimens (15-17 hours)under an argon atmosphere with temperatures ranging from 1320° C. to1430° C., theoretical densities of 93-96% were achieved with the MgOflux.

Forming Dense High-alumina Bodies

[0029] The present invention relates to a method for producing densebodies having a high-alumina content from powder alumina precursors.More particularly, the present invention relates to a technique for thesintering of high-alumina bodies at lower temperatures to form densehigh-alumina bodies having superior physical properties, as shownschematically in FIG. 1. In general, the first step in thelow-temperature production of high-alumina bodies is to blend ahigh-alumina green powder. The high-alumina green powder is blended fromcalcined alumina powder, with additions of about 1-10 wt. magnesia (or amagnesia-former standardized to about 1-10 wt. % magnesia) and about1-10 wt. % titania. The magnesia addition may be conveniently achievedthrough the addition of a magnesia-former, such as MgCO₃, the firing ofwhich readily forms magnesia upon heating according to the relation:

MgCO₃.MgO+CO₂

[0030] For the convenience of the reader, “magnesia” hereinbelow will betaken to refer to both MgO and any MgO forming material standardized toproduce MgO. Likewise, “titania” will refer to TiO₂ and any TiO₂ formingmaterial standardized to produce TiO₂. Preferably, about 4 wt. %additive mixture is added to the calcined alumina powder to constitutethe green precursor. Also preferably, the ratio of magnesia to titaniain the additive portion is about 50:50, and more preferably the ratio isabout 42:58. The precursor powders are preferably mixed by wet ballmilling with alumina media, although any convenient ceramic powdermixing technique may be chosen. Also, binder phase such ascarboxymethylcellulose (CMC), may be added to the green powder,depending upon the requirements of the pressing and firing parametersnecessary to produce the desired high-alumina body.

[0031] The dried green powder is then sieved and formed into a greenbody having the desired shape. The green body is then baked to removeexcess moisture and the binder phase (if any) and then fired.Preferably, the alumina body is fired in air to about cone 13 to achievefull sintering and densification. It should be noted that the conesystem of measurement combines firing time and temperature to achievewhat is essentially a measure of a system's energy state, i.e. theenergy at which a cone of a specific composition softens and deforms.Cone 13 is roughly analogous to firing to about 1250° C. for about 2hours. The baking and firing phases may be performed separately, or aspart of one continuous process.

[0032] One alternative to the firing step is passing the green particlesthrough a heat source, such as a flame or laser. If the green particlesare rapidly passed through a sufficiently intense hot zone, rapidsintering may be induced. Moreover, if the green particles are passedthrough the hot zone under weightless or quasi-weightless conditions(such as aspiration), surface tension effects from the molten binderphase will cause the heated particles to take on a substantiallyspherical shape as they sinter.

[0033] Preferentially, CMC in a 3% aqueous solution is used as thebinder. In other contemplated embodiments, other convenient organicbinders may be used. Likewise, while the preferred concentration of CMCis 3% in aqueous solution, any convenient concentration of CMC capableof producing a crushable solid residue may be chosen.

[0034] The purity of the green powder precursor materials are notcritical to the present invention, although if the production of ahighly pure high-alumina body is desired, the use of high purity greenpowder precursor materials may likewise be desirable. If the purity ofthe resultant high-alumina bodies is not a consideration, green powderprecursors of any desired purity level may be selected.

[0035] In the preferred embodiment, the calcined alumina precursors werechosen from powders having a particle size of about 1 micron or less,but precursor particles of any convenient size may be selected. Thelow-temperature high-alumina sintering process of the present inventionis not especially sensitive to precursor particle size, with the size ofthe precursor particles primarily influencing slurry mixing conditionsand green body pressing/forming parameters. However, it is generallypreferable for the mean particle size of the additives to be about equalto or smaller than that of the calcined alumina.

Properties of Low-fired High-alumina Bodies

[0036]FIG. 2 is a table illustrating the basic physical properties oflow-fired high-alumina material made by the above process, while FIG. 3is a table showing the effects of various hostile chemical environmentsof the same low-fired high-alumina material. In addition, high-aluminabodies produced by the above process have a number of interestingproperties, including: substantially full density; increased resistanceto chemical attack over a very broad pH range; the substantial absenceof a secondary glassy phase (i.e., they are non-vitreous); substantiallyuniform and linear thermal expansion; optical translucence;high-temperature corrosion resistance; and substantially uniform grainsize.

[0037] Of particular interest is the pH range over which the low-firedhigh-alumina material is resistant to chemical attack, as illustrated inFIG. 3. Bodies made of the low-fired high-alumina material have beensubjected to pH conditions ranging from extremely alkaline (concentratedhot NaOH) to extremely acidic (hot concentrated HF, H₂SO₄ and hot H₂gas) with minimal corrosive effects. The high-alumina bodies are evenresistant to dissolution and/or corrosion from prolonged immersion inmolten aluminum.

[0038] Moreover, the above process produces high-alumina pieces having avery low rate of defect, allowing net shape formability throughconventional green body forming and firing means. Further, thehigh-alumina pieces formed by the above process consistently exhibit asuperior surface finish of about 8 rms. The savings (both in energycosts and increased furnace life), the uniform and linear thermalexpansion, substantially uniform grain size, low defect rate, andsuperior surface finish make the formation of low-fired high-aluminamaterial by the above process very attractive from a manufacturingstandpoint. Low-fired high-alumina bodies of the do not require kilnfurniture or spacers for separation and may be stacked directly incontact with one another for firing without risking fusing or otherfiring defects.

Low-fire High-alumina Spray Coatings

[0039] Low-fired high-alumina material made by the above process mayalso be applied as a thermal spray material coating via techniques suchas subsonic plasma coating or high velocity oxygenated fuel (HVOF)means. Thermal spray coatings of a low-fired high-alumina material ofthe present invention provide a tough ceramic wear resistant andcorrosion resistant coating layer suitable for mechanical or electronicapplications without sensitivity to the application technique. FIG. 4tabulates some of the properties of low-fired high-alumina thermal spraycoatings.

Metal Matrix Composites Containing Low-fire High-alumina Materials

[0040]FIG. 5 presents some properties of metal matrix composite (MMC)materials made using the low-fired high-alumina material of the presentinvention (in spherical form) as a reinforcement phase. In thisembodiment, the metal matrix was aluminum, although any convenient metalmatrix may be reinforced using the present low-fired high-aluminamaterial. The high resistance to dissolution in molten aluminumexhibited by the present low-fired high-alumina material allows MMCsmade therefrom to be made by a casting process instead of the moreexpensive cold pressed powder metallurgical process. In addition, MMCsmade with the present low-fired high-alumina material enjoy theadvantages of enhanced welded joint integrity, an expanded heattreatment range and a higher manufacturing throughput.

[0041] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only the preferred embodiment has been shown and described and thatall changes and modifications that come within the spirit of theinvention are to be desired to be protected.

What is claimed is:
 1. A method for producing high-alumina bodies atreduced sintering temperatures, comprising the steps of: a) providing analumina powder precursor; b) adding about 1-10 wt. % magnesia powderprecursor and 1-10 wt. % titania powder precursor to the alumina powderprecursor to make a green powder precursor; c) mixing the green powderprecursor; d) pressing a green body from the green powder precursor; e)removing residual moisture and organic material from the green body; andf) firing the green body to about cone
 13. 2. The method of claim 1further comprising the step of between b) and d), adding a binder. 3.The method of claim 2 wherein the binder is an aqueous solution of about3% carboxymethylcellulose.
 4. The method of claim 1 wherein the greenbody is fired in air.
 5. The method of claim 1 wherein magnesia andtitania are added in a substantially 50:50 ratio.
 6. The method of claim1 wherein magnesia and titania are added in a substantially 42:48 ratio.7. The method of claim 1 wherein about 2 wt. % magnesia and about 2 wt.% titania are added.
 8. The method of claim 1 wherein mixing isaccomplished by wet ball milling with alumina media.
 9. The method ofclaim 1 further comprising the step of between b) and d), adding a 3%aqueous solution of carboxymethylcellulose; wherein the green body isfired in air; wherein about 2 wt. % magnesia and about 2 wt. % titaniaare added in a substantially 42:48 ratio; and wherein mixing isaccomplished by wet ball milling with alumina media.
 10. A method forproducing high-alumina bodies having enhanced chemical stability atreduced sintering temperatures, comprising the steps of: g) providing analumina precursor; h) adding about 1-10 wt. % magnesia precursor and1-10 wt. % titania precursor to the alumina powder precursor; i) mixingthe alumina precursor; j) forming the alumina precursor into a desiredshape; and k) firing the alumina shape to produce a substantiallynon-vitreous high alumina body.
 11. The method of claim 10 wherein thehigh alumina body has a substantially uniform grain size.
 12. The methodof claim 10 wherein the alumina precursor is a powder and wherein thealumina precursor is formed into a desired shape by pressing.
 13. Themethod of claim 10 wherein the alumina precursor is a slurry and whereinthe alumina precursor is formed into a desired shape by casting.
 14. Themethod of claim 10 wherein the alumina precursor is a slurry and whereinthe alumina precursor is formed into a desired shape by spraying. 15.The method of claim 10 wherein the substantially non-vitreous highalumina body is part of a metal matrix composite.
 16. The method ofclaim 15 wherein the metal matrix is aluminum.
 17. A high alumina bodyformed by the steps of: aa) providing an alumina precursor; bb) addingabout 1-10 wt. % magnesia precursor and 1-10 wt. % titania precursor tothe alumina powder precursor; cc) mixing the alumina precursor; dd)forming the alumina precursor into a desired shape; and ee) firing thealumina shape to produce a substantially non-vitreous high alumina body.18. The body of claim 17 further comprising the step of between bb) anddd), adding an approximately 3% aqueous solution ofcarboxymethylcellulose; wherein the green body is fired in air; whereinabout 2 wt. % magnesia and about 2 wt. % titania are added in asubstantially 42:48 ratio; and wherein mixing is accomplished by wetball milling with alumina media.
 19. A chemically resistant high aluminabody formed by the steps of: gg) providing an alumina precursor; hh)adding about 1-10 wt. % magnesia precursor and 1-10 wt. % titaniaprecursor to the alumina powder precursor; ii) mixing the aluminaprecursor; jj) forming the alumina precursor into a desired shape; andkk) firing the alumina shape to produce a substantially non-vitreoushigh alumina body.
 20. The body of claim 19 further comprising the stepof before ii) adding an approximately 3% aqueous solution ofcarboxymethylcellulose; wherein the high alumina is fired in air;wherein about 2 wt. % magnesia and about 2 wt. % titania are added in asubstantially 42:48 ratio; and wherein mixing is accomplished by wetball milling with alumina media.